The present application relates to a method of manufacturing a magnetic disk.
In order to realize a large capacity magnetic storage device, it is required to achieve further high-density. In hard disk drives, which are typical magnetic storage devices, those with surface recording density of higher than several tens gigabit per square inch have already been commercialized.
FIG. 10 is a schematic cross-sectional view of a typical hard disk medium. This hard disk medium is composed by sequentially providing a magnetic layer 2, a protective layer 3, and a lubricant layer 4 on a flat surface of a substrate 1 formed of a Al substrate provided with a Ni layer on the surface thereof. Annular recording tracks are disposed in a flat and continuous magnetic layer concentric around the central axis illustrated with a chain line c of the center hole 5. The recording tracks are not physically isolated from the adjacent ones and formed as a continuous area.
Therefore, in such a hard disk medium, the density in the radial direction, namely the track pitch, affects the S/N ratio of a reproduction signal.
In such a case in which the configuration having the adjacent recording tracks in the continuous area is adopted, the track width (hereinafter simply referred to as a magnetic recording head width) Ww of a magnetic gap of the magnetic recording head needs to be selected to be narrower than the recording track width W (Ww<W) considering the effect to the S/N ratio of the reproduction signal.
That is, the width of a recording mark recorded by the magnetic recording head becomes, in practice, wider than the magnetic recording head width Ww because of the blurring phenomenon caused by the leakage magnetic field in the radial direction of the hard disk medium derived from the magnetic gap of the magnetic recording head. Therefore, the width of the recording mark is composed of a part (with substantially the same width as the magnetic recording head width) formed by the magnetic recording head and a part formed by the blurring phenomenon, and it can be said that a correct reproduction signal cannot be obtained from the part of the recording mark formed by the blurring phenomenon.
Therefore, if the magnetic recording head width Ww is set to be substantially the same as the recording track width W, the magnetic recording head writes a superfluous signal in the adjacent recording track, which causes corrosion of the recording mark formed on the adjacent track.
Then, if such corrosion of the recording mark occurs, the S/N ratio of the reproduction signal is degraded when the reproduction head performs detection of the leakage magnetic field, namely reproduction of the recorded information data of this recording mark.
Therefore, the magnetic recording head width Ww needs to be selected to be narrower than the recording track width W (Ww<W) as described above.
Meanwhile, in order to obtain high-sensitive reproduction signal from the leakage magnetic field of the recording mark, it is necessary to avoid detecting the leakage magnetic field from the part formed by the blurring phenomenon in the recording process as described above. Accordingly, the width Wr of the reproduction head needs to be equal to or even narrower than the width of the recording mark. In particular, since the center of the recording track cannot always be traced because of the servo error in the reproduction operation, the reproduction head width Wr needs to be selected taking the error into consideration.
However, since an only small reproduction signal, which causes the S/N ratio to decrease, is obtained with the narrow reproduction head width Wr, it is preferable to make the width Wr as large as possible.
In consideration of the above, the relationship between the reproduction head width Wr, the magnetic recording head width Ww and the recording track width W will be selected as follows.
Wr<Ww<W
In other words, it is difficult to form the recording mark using the whole of the recording track width W, and on the other hand, it is difficult to use the whole width of the recording mark thus formed as the object of reproduction.
Therefore, a method called discrete track recording (DTR) is presently proposed as the density growth technology for hard disk drives (see, for example, IEEE Transaction on Magnetics, Vol. 40, No. 4, Jul. 2004, pp. 2510-2515, hereinafter referred to as a first document).
According to the DTR, the problem regarding the recording track pitch described above can be alleviated. Specifically, it is a hard disk medium provided with a physical separation groove between the adjacent recording tracks to separate the adjacent recording tracks from each other, the groove being formed so that the leakage magnetic field from the inside of the groove does not reach the reproduction head, namely so as to have a groove depth and a groove shape enough for preventing the reproduction head from detecting the leakage magnetic field.
According to the DTR, since consideration of the blurring phenomenon is not necessary in the hard disk medium provided with the separation groove formed between the adjacent recording tracks, the magnetic recording head width Ww can be made larger than the width of a land defined between the grooves of the recording track, and accordingly, the recording mark can be formed to have the width identical to the whole width of the land between the separation grooves.
Further, at the same time, since the consideration of the blurring phenomenon is not necessary, the magnetic reproduction head width Wr can be made larger than the whole width of the land between the separation grooves, namely the land width. Accordingly, the whole width of the land can absolutely be used for reproduction even with the servo error.
In other words, in the DTR configuration, since the width of the recording mark is determined by the land width, and the recording mark can be formed to have large width, the S/N ratio to the track pitch can be increased.
Incidentally, in a past process of manufacturing the magnetic disk substrate with such a DTR structure, namely the concavo-convex separation groove, the magnetic disk is formed by etching the magnetic film provided on the magnetic disk substrate applying a lithography process using an electron beam or a lithography process of a nanoimprint method to separate the magnetic film with the separation groove, and further filling the separation groove with a planarizing layer made of a silicon oxide film as described in the first document (see TMRC 2004 Paper F5 Submitted for Publication in IEEE Transaction on Magnetics, pp. 1-6).
However, there has been a problem of remarkably lowering the productivity in manufacturing the magnetic disk substrate by processing every magnetic disk substrate with the electron beam lithography process or the nanoimprint process.